One of the most effective materials from which the media of such total heat energy exchange wheels is made is asbestos in the form of alternately flat and corrugated asbestos paper sheets, preferably impregnated with a hygroscopic salt such as lithium bromide or lithium chloride for improved transfer of moisture. Although such heat and moisture exchange media have also been made of cellulosic paper, asbestos has been considered generally superior to paper or other material, at least in terms of the latent heat and moisture exchange rates and efficiencies of comparably sized media. See, for example, U.S. Pat. No. 3,398,510 (Pennington) in which a comparison is made between the characteristics of cellulosic paper and asbestos paper in such wheels, albeit as used for moisture transfer, rather than for total heat exchange. However, question has arisen regarding whether the use of such asbestos material in direct contact with the stream of makeup air which is being supplied to living or working quarters may permit even extremely minute quantities of harmful asbestos fibres to be introduced into environmental air being breathed by human beings. Accordingly, a need has arisen for, and it is therefore a principal object of the invention to provide an efficient total heat energy exchange apparatus whose heat exchange media is made from materials other than asbestos.
However, it is also a purpose to provide a total energy exchange media made from materials such that the wheel will have its own desirable moisture and heat energy absorption and exchange characteristics for a wide range of applications, and in wheels of various sizes from as small as about 28 inches to as large as about 12 feet in diameter, considering that the wheel thickness or depth should be retained within a reasonable limit of, say, less than 2 feet.
It is further intended by the invention to establish parameters for use in connection with such wheels regarding the length of the airstream passages extending through the depth or thickness of the wheel with relation to the diameter of such air passages, thus to provide optimum sensible and latent heat transfer efficiencies with minimal fluid flow losses and consequent minimal air pressure drop across the wheel, yet with openings whose size is adequate to minimize the possibility for clogging of the passages by flowing particulate matter.
Because it is inexpensive and does have indicated desirable heat exchange characteristics, the present invention further contemplates that such media might be made wholly or partially of cellulosic paper, provided the composition of the paper, either by itself or together with any other materials incorporated in the media, is such as will impart the desired operating characteristics, yet resist or retard fire and wheel distortion during both fabrication and use, and have adequate wet strength so as not to sag or tear apart when exposed to high humidity conditions. To impart such improved heat exchange characteristics as well as strength to the wheel structure, the invention also contemplates that an appropriate mass of metal might also be incorporated into the wheel structure.
Further, a wheel having such improved media must be constructed so as to adequately retain the media in place during use, and the media must be durable to provide reasonably long-life service under the normally to be encountered conditions of heat and moisture. Of course, the wheel should be relatively inexpensive to fabricate and maintain.
Total heat energy recovery and exchange devices having regenerative exchange wheels whose packing or media is made entirely of cellulosic paper, are known. For example, in Canadian Patent No. 629,879 (Munters) wheels made of spirally wound, alternately flat and corrugated webs of paper for the purpose are described.
Devices for the recovery and exchange of only sensible heat are commonly provided with all-metal foil wheels formed by spirally winding either stainless steel foil or aluminum foil about a hub. See, for example, U.S. Pat. No. 3,702,156 (Rohrs et al), and Canadian Patent No. 629,879 (Munters).
Regarding previous all-paper media, although they have the advantage of being far less expensive than those made of asbestos paper, they do not always have fire retardant and bacteriostatic characteristics as are desirable, and are difficult to manufacture especially in spirally wound form and having large size on the order of, say, 12 ft. in diameter. In addition, it has been believed necessary to either coat or especially treat the paper material after its manufacture to improve its sensible heat and/or latent heat and/or moisture transfer capabilities, or to impart strength to withstand the temperature and humidity conditions to which the paper will be subjected during use. See, for example, U.S. Pat. No. 3,664,095 (Asker et al). Moreover, in use the devices are exposed to dynamic forces of the fluids which pass through it, which promotes wear and tear, and they are sometimes impinged by particulate matter such as dust carried by the flowing fluid, which also tends to clog the narrow air passages through the wheel.
U.S. Pat. No. 3,155,153 (Axelson) describes a technique for impregnating the end faces of corrugated paper wheels (or wheels of other organic or inorganic materials) with a stiffening material for imparting strength to resist deterioration by such dynamic fluid forces and particle impingement, and which also serves to bond together the successive convolutions of the wound corrugated paper. Further with regard to the manner of construction of such wheels, the above-referred to U.S. Pat. No. 3,702,156 (Rohrs et al) describes an effective technique for fabricating sensible heat transfer wheels in which, after winding, the spirally wound continuous strips of alternately flat and corrugated metal foil are grooved along their end faces to receive an appropriate number of radial spokes to form a wheel which is entirely flush across its end faces, as is a feature of the present invention. The flush disposition of the spokes and media at the end faces is advantageous in that it improves the strength of such wheels, and also facilitates the forming of an effective air seal at the interfaces between the moving wheel and the ends of the stationary airstream duct dividers which usually form a part of the frame within which the wheel is mounted.
Briefly describing the presently preferred embodiment of an air-to-air total heat energy recovery and transfer wheel in accordance with the invention, respective continuous flat and corrugated strips of aluminum foil, together with a pair of paper strips respectively conforming to, and disposed on each side of either the corrugated foil or the flat foil strip, are concurrently and spirally wound on to a metal hub. In an alternative embodiment, an all-paper media is provided by winding single-faced corrugated paperboard on the hub.
In all embodiments it has been found that, to impart the desired characteristics, the paper is preferably a fully bleached Kraft paper of about 45 pound density (i.e., 45 lbs. per 500 sheets each measuring 24 inches .times. 30 inches), having thickness of from about 4 mils to about 6 mils, preferably 0.0045 inch (41/2 mils), and whose ultimate composition is 84% fibres and 16% contained salts, the salts being 90% ammonium sulfide and 10% diammonium phosphate. Although stainless steel or other sheet metal might be used, it has been found that both the flat and the corrugated metal strips or webs as are incorporated in the preferred media are preferably of a high purity aluminum of medium hardness.
The size of the opening of the somewhat circular air passages formed by the sine-shaped corrugations of the corrugated web in the media is from about 0.055 inch to about 0.120 inch, and the length of the air passages is from 100 to 400 times such corrugated height. More particularly, where the wheel is made from two plies of metal foil and two paper plies, as in the preferred embodiment of the invention, it has been found that the height of the corrugations should be from about 0.055 inch to about 0.065 inch, preferably 0.060 inch, and that the length of the air passages should be from about 125 to 150, preferably about 135, times such height (i.e., about 8 inches) whereas in the case of the all-paper embodiment, the corrugation height should be from about 0.06 inch to about 0.12 inch, preferably 0.10 inch, and the length of the passage is preferably 200 times that height (i.e., about 20 inches). In this regard, it has been found that the maintenance of such L/D ratios, (where L is the common length of the tubular passages formed by the corrugations through the wheel depth, and D is the passage diameter corresponding to the flute height of the corrugations) is important for the attainment of high sensible and latent heat energy recovery effeciencies with minimal pressure drop across the wheel.
In the modified form of the invention in which the total heat energy transfer media is made entirely from strips of alternately flat and corrugated paper, the strips are preferably adhesively secured together by a silicate adhesive as is conventional in the making of single-faced corrugated paperboard. Thus, it is convenient to form the media from a web of previously made single-faced corrugated paperboard. It will be noted that, in addition to its latent and sensible heat transfer capabilities, the specified paper of which the media is made, so treated as aforesaid, has a fire-resistant characteristic and is also bacteriostatic.
In such all-paper embodiment, the successive convolutions of the corrugated paperboard web need not be adhered to each underlying convolution of the web. However, to impart stiffness to the media, render it bacteriocidal, and to improve the sensible heat transfer capability of the wheel, a dryable neoprene rubber, which also functions as an adhesive, may be applied between the convolutions, either in several laterally spaced apart strips continuously applied as the paper is wound around the hub, or preferably by an airless spray application, or a dipping or painting-on application of the liquid neoprene to the end faces of the wheel after the corrugated paperboard material has been wound tightly on the hub. Thus, whether applied by dipping or spraying into the end faces or as continuous strips during the winding of the paper material, the adhesive is deposited in the form of at least a pair of adhesive strips extending inwardly at or near the end faces of the wheel. In this regard, an airless spray application of the neoprene penetrates the end faces of the wheel to a depth of about 5 inches or more. When applied by roller application during the winding of the corrugated paper material, one more strip of adhesive is preferably located centrally within the width of the corrugated paper web, such being desirable in wheels having considerable thickness.
To render the wheel bacteriocidal, such liquid neoprene may be similarly applied to the end faces of the preferred, combination metal foil-and-paper wheel.
In a still further modified form of the invention, a single, thin metal foil strip, preferably aluminum foil, of the same or slightly greater width as the corrugated paperboard web, is concurrently wound for the purpose of improving the sensible heat transfer capability of the wheel for particular wheel applications. Although not always necessary, in this embodiment, strips of adhesive are preferably disposed on both sides of the metal foil strip so that the interleaved metal foil is adhesively secured to both the flat paper ply and the corrugated ply of the wound, single-faced corrugated paperboard material.
With reference to any of the described embodiments, the winding of the media around the wrapping hub may be discontinued when a selected radius of from about 12 inches to about 24 inches has been achieved so that an annular metal band, having width equal to the depth of the wheel, may be tightly attached around the media at that location. Spiral winding of additional strips or webs of the media-forming material is then continued around the exterior of the annular metal band until a predetermined additional radius of the wheel is reached, whereupon a similar annular metal band is fastened tightly around the second media area. Any number of such intermediately located, annular metal bands can be applied at each preselected interval of wheel radius. When the desired wheel diameter is ultimately attained, a somewhat heavier peripheral metal rim is fastened tightly around the built-up structure.
To retain the wound media and annular metal bands in place, radially extending metal spokes are attached between the hub and peripheral rim on both sides of the wheel. Both for the purpose of providing a flush face at either side of the wheel and for imparting rigidity to the face of the spirally wound media itself, each radial spoke is placed within a milled radial slot in the face of the wheel, the slot extending between the hub and the peripheral rim. Each spoke fits closely within its associated radial slot, and is welded to the metal hub, the metal peripheral rim, and to each of the intermediate annular metal bands of the wheel. Where the media itself includes metal foil, the spokes are additionally welded to the end faces of the foil convolutions. Where the media is all paper, each spoke is coated with an adhesive, such as an epoxy resin, before it is placed within its media slot so that each ply of the corrugated paper media will be adhesively secured thereto.
The referred to preferred L/D ratio of from about 125 to 200 has been found to be optimum from the standpoint of total heat energy recovery efficiencies, cost, and convenience of handling the resulting size media during fabrication. That is, it has been found that the wheel dimensions resulting from such selected optimum ratio cause the wheel to be less cumbersome to build. Moreover, to avoid clogging by the deposit of particulate matter under normally encountered conditions, the diameter of each air passage through the wheel should be not less than about 0.055 or 0.06 inches.
It should also be noted that, in any of the described embodiments, the sensible heat transfer efficiency of the matrix is somewhat higher than its latent heat transfer efficiency. Thus, as represented on a psychrometric chart the three significant operating points representing the temperature and moisture conditions of the air at the respective locations of atmospheric supply, room air supply, and room air exhaust in a system incorporating the wheel, do not lie on a straight line. This may be advantageous under certain conditions of operation.